The subject matter of this application is related to the subject matter of British Patent Application No. GB 0028733.4, priority to which is claimed under 35 U.S.C. xc2xa7119 and which is incorporated herein by reference.
1. Field of the Invention
This invention relates to switched reluctance drive systems. In particular, it relates to such systems where the controller is able to minimize the ripple current in the DC link capacitor.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, xe2x80x9cThe characteristics, design and application of switched reluctance motors and drivesxe2x80x9d by Stephenson and Blake, PCIM""93, Nxc3xcrnberg, Jun. 21-24, 1993, incorporated herein by reference. FIG. 1 shows a typical polyphase switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The input DC power supply 11 can be, for example, a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. Its output may also be used to generate a speed feedback signal.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the xe2x80x9cDC linkxe2x80x9d of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the xe2x80x9cDC link capacitorxe2x80x9d, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called xe2x80x9cripple currentxe2x80x9d) that cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel, and where parallel connection is used some of the elements may be distributed throughout the converter. The cost and/or size of this capacitor can be important in installations which are sensitive to drive cost and/or the space occupied by the drive, for example in aerospace or automotive applications.
The switched reluctance drive is essentially a variable-speed system and is characterized by voltages and currents in the phase windings of the machine that are quite different from those found in traditional, sinusoidally fed, types of machines. As is well known, there are two basic modes of operation of switched reluctance systems: chopping mode and single-pulse mode, both of which are described in the Stephenson paper cited above. FIGS. 3(a)-3(c) illustrate single-pulse control, which is normally used for medium and high speeds in the speed range of a typical drive. FIG. 3(a) shows the voltage waveform typically applied by the controller to the phase winding. At a predetermined rotor angle, the voltage is applied by switching on the switches in the power converter 13 and applying constant voltage for a given angle xcex8c, the conduction angle. The current rises from zero, typically reaches a peak and falls slightly as shown in FIG. 3(b). When the angle xcex8c has been traversed, the switches are opened and the action of energy return diodes places a negative voltage across the winding, causing the flux in the machine, and hence the current, to decay to zero. There is then typically a period of zero current until the cycle is repeated. It will be clear that the phase is drawing energy from the supply during xcex8c and returning a smaller amount to the supply thereafter. FIG. 3(c) shows the current that has to be supplied to the phase winding by the power converter and the current that flows back to the converter during the period of energy return. Instead of opening both switches simultaneously, it is well known that there are advantages in opening one switch in advance of the other, allowing the current to circulate around the loop formed by the closed switch, the phase winding and one of the diodes. This is known as xe2x80x9cfreewheelingxe2x80x9d. It is used for various reasons, including peak current limitation and acoustic noise reduction.
At zero and low speeds, however, the single-pulse mode is not suitable, due to the high peak currents that would be experienced, and the chopping mode is used to actively control the phase winding current. There are two principal variants of the chopping mode. When the current reaches a predetermined level, the simplest method of chopping is to simultaneously open the two switches associated with a phase winding, e.g. switches 21 and 22 in FIG. 2. This causes energy to be returned from the machine to the DC link. This is sometimes known as xe2x80x9chard choppingxe2x80x9d. The alternative method is to open only one of the switches and allow freewheeling to occur. This is known as xe2x80x9cfreewheel choppingxe2x80x9d or xe2x80x9csoft choppingxe2x80x9d. In this mode of control, no energy is returned to the DC link from the phase winding, except at the end of the conduction period, when both switches are opened to finally extinguish the current.
With any chopping scheme, there is a choice of strategy for determining the current levels to be used. Many such strategies are known in the art. One commonly used scheme includes a hysteresis controller that enables chopping between upper and lower currents. A typical scheme is shown in FIG. 4(a) for hard chopping. At a chosen switch-on angle xcex8on (which is often the position at which the phase has minimum inductance, but may be some other position), the voltage is applied to the phase winding and the phase current is allowed to rise until it reaches the upper hysteresis current Iu. At this point both switches are opened and the current falls until it reaches the lower current Il and the switches are closed again, repeating the chopping cycle. FIG. 5(a) shows the corresponding phase current waveform for a hysteresis controller using freewheeling. The reduction in chopping frequency is immediately evident.
It should be noted that if the machine is generating rather than motoring, the phase current may rise during freewheeling. Soft chopping can still be used by alternating the power circuit states between freewheeling (with one switch open) and energy return (with both switches open). The techniques described hereafter apply equally to motoring and generating modes of operation.
The supply currents flowing in the DC link due to the phase currents in FIGS. 4(a) and 5(a) are shown in FIGS. 4(b) and 5(b) respectively. In each case, the DC link capacitor supplies a proportion of the ac component of these waveforms. (Note that these figures are idealized, since the capacitor must have zero mean current and, in practice, the behavior of the currents in the presence of supply resistance, capacitor impedance and inductance is considerably more complex.) The capacitor current in the hard chopping case has both a higher frequency and a higher root mean square (rms) value. In the freewheeling case the current is reduced in both frequency and rms magnitude. The benefits of this with respect to capacitor rating are discussed in, for example, U.S. Pat. No. 4,933,621 (MacMinn), which is incorporated herein by reference.
While the hard and soft chopping modes have been described in the context of current control, it should be noted that they can also be used in conjunction with a voltage control system, where the average voltage applied to the phase winding is actively controlled. For example, a pulse width modulation (PWM) scheme may be applied to the switches 21 and 22, as is well known in the art.
The discussions so far have ignored the problem that occurs when the contributions of two or more phases are considered. Whenever two or more phases are operated together, the currents associated with the individual phases are added to give the total DC link current.
Two or more phases operating together can occur in many different systems. Although in 2-phase systems it is usual only to operate the phases alternately, U.S. Pat. No. 5,747,962, incorporated herein by reference and commonly assigned to the present assignee, discloses a method of operating both phases simultaneously over part of the electrical cycle of the machine. In 3-phase machines, it is possible to operate by exciting Phase A alone, then Phase B alone, then Phase C alone. However, to improve the torque output of the machine, advantage is often taken of the fact that the torque productive portions of each phase cycle overlap, so an excitation pattern of A, AB, B, BC, C, CA, A . . . is normally used. Similarly for 4-phase machines, there are normally always two phases producing torque in the required direction, so phases can be energized in pairs: AB, BC, CD, DA, AB . . . Corresponding rules apply for higher phase numbers, in which it is possible to use three or more phases for at least part of the electrical cycle.
When two or more phases are being used simultaneously, the effect on the capacitor current depends on the chopping control strategy adopted. Ideally, the energy returned by one phase at switch off should be channeled into an incoming second phase, but it is found that this is impossible to achieve with the normal hysteresis controller. For example, FIG. 4(a) and FIG. 5(a) show that neither the frequency nor the duty cycle of the chopping waveform are constant over the conduction angle, but vary as the inductance of the phase winding varies with rotor position. It is clear that it is not possible to achieve cancellation of the capacitor current whatever phase difference there is between the two currents with this type of controller.
The hysteresis controller is an example of the many xe2x80x9cfrequency wildxe2x80x9d controllers which exist. These attempt to keep one parameter (here phase current) constant at the expense of the chopping frequency. A different type of chopping controller is a fixed frequency controller, in which the chopping frequency is kept notionally constant at the expense of, say, average phase current. A typical fixed frequency controller has a fixed (or at least controlled) frequency clock which triggers the closing of the switches in the power converter 13. One or both switches are opened when the phase current reaches its target value, allowing the current to fall until the switches are closed again in response to the clock signal. Controlling a second phase from the same clock is relatively simple but it will be clear that switching the two phases together has the immediate effect of approximately doubling the magnitude of the current excursions on the capacitor (depending on the duty cycle).
A typical chopping current waveform for a phase winding controlled by a peak current, fixed frequency controller is partly shown in FIG. 6(a). The phase winding is initially switched on by connecting it to the DC bus in the usual way. When the current reaches a pre-determined peak level, the controller switches the winding into freewheel or energy recovery mode, depending of the abilities of the power converter in use. (In FIG. 6(a), freewheeling is used for illustration.) The current falls at a rate determined by the voltage applied (if any), the rate of change of inductance of the phase winding, the voltage drop (iR) across the winding and the voltage drops across the diode(s) and switch. Assuming motoring operation, the current in the freewheeling interval will fall, as shown, and the current controller then triggers the reconnection of the phase winding to the DC link by closing the switch(es), forcing the current up again to the peak level, whereupon the cycle repeats. As will be seen, the frequency of the switching is now constant, while the current excursions from the peak may change from one end of the excitation block to the other. The period of the switching frequency is marked as T in FIG. 6(a), and the duty of the cycle is defined as the ratio of on time to the duration of the period.
FIG. 6(b) shows the supply current for one phase winding and FIG. 6(c) the supply current for a second phase. If the switch-on points of both phases are simultaneous, the supply currents will add to give a peak equal to 2*Ipk and the corresponding capacitor ripple current at the chopping frequency will be high.
It is known specifically for 4-phase systems having two phases always energized together to use a single clock signal with a unity mark: space ratio and to initiate the turn-on of one phase from the rising edges, and the other phase from the falling edges, of the clock signal. This interleaves the two phase currents in a fixed manner and results in some reduction of capacitor ripple current. It is not completely successful, however, in that, as the duty cycle of one phase changes relative to the other, the capacitor current increases. The interleaving of the phase currents is fixed and cannot cope with such variations. This requires an increase in the rating of the capacitor, resulting in increased cost and/or size. In addition, the energy loss in the capacitor can represent a significant fraction of the overall energy loss in some systems, particularly low-voltage, high-current systems, leading to a noticeable reduction in efficiency of the drive.
There is, therefore, a need for a solution to the problem of minimizing capacitor ripple current for a system with any number of phases and any pattern of excitation in which two or more phases can be excited coincidentally.
In one particular form, the invention includes dynamically tuning the switching of the incoming phase winding of a switched reluctance machine so as to minimize the capacitor current. This is done by dynamically selecting the phase angle of the chopping clock of the incoming phase with respect to the phase already chopping. The invention can be implemented in a number of ways, which broadly fall into the two categories of open-loop and closed-loop implementations.
The chopping waveforms arising from the excitation of two phases simultaneously can be used to minimize the current ripple by setting a phase shift between the chopping clock signals that control the two phases.
The setting of the phase shift can be preset or it may be dynamic open loop or closed loop. If it is variable under open or closed loop control, the value of the phase shift can be based on a parameter of the machine. For example, a demand and speed can be used for open loop control. For closed loop control, a signal representative of the current ripple itself can be used to derive a value of phase shift that will minimize the capacitor current ripple.